1. Field of the Invention
The present invention relates to an apparatus for fixing a half mirror of an optical pickup for use in a disc player, for example, a DVD player.
2. Description of the Related Art
FIG. 8 shows an example of a conventional optical pickup. This optical pickup is adapted so that a half mirror 3 is disposed in a light passing hole 2 of a pickup body 1 in such a way as to be in an inclined state. An objective lens OL, and a set of a collimator lens QWP and a photodiode PD are respectively disposed at both sides of the pickup body 1 so that the half mirror 3 is placed therebetween. A semiconductor laser LD is provided in a branch hole 4 formed on a side surface of the pickup body 1. Laser light is projected from the semiconductor laser LD through the half mirror 3, the collimator lens QWP, and the objective lens OL onto a disc D. Then, light reflected there from is received by a photodiode PD through the half mirror 3. Thus, information recorded onto the disc D is read.
Conventional techniques for fixing the half mirror 3 are described in JP-A-60-80521U. An example of an apparatus using such techniques is described hereinbelow with reference to FIGS. 9 to 12. This example is formed so that the shape of the transversal section of a part of the light passing hole 2, which part stores the half mirror 3, is a rectangular, that the left-hand side surface 5a and the left-hand side surface 5b among four inner surfaces 5a to 5d of the rectangular part, and that triangular plates 6 are formed in such a manner as to be integral with both the left-hand inner side surface 5a and the right-hand inner side surface 5b, which face the side edges 3a and 3b, respectively. The half mirror 3 is put on slope-like top surfaces 6a of both the triangular plates 6. An innermost end edge 3c of the half mirror 3 is made to abut against a slope-like abutting face 7 formed on the bottom end of one 5c of the remaining inner side surfaces 5c and 5d. Moreover, a pair of a left-hand side concave groove and a right-hand side concave groove 8, which communicate with an opening portion 2a of the light transmitting hole 2, are formed on both sides of the other side surface 5d, respectively. A fastening device 9 constituted by a leaf spring is placed along the other side surface 5d. 
As shown in FIGS. 11 and 12, the fastening device 9 comprises a fastening device body 9a placed along the latter side surface 5d, a pair of left-hand and right-hand leg plate portions 9b each of which has a nearly-L-shaped transversal section and is extended from a corresponding one of both the side edges of the fastening device body 9a and fitted into a corresponding one of the concave grooves 8, a pair of left-hand and right-hand presser plate portions 9c that are extended from the fastening device body 9a and elastically abut against the surface of the half mirror 3, a pair of left-hand and right-hand engaging claws 9d that are bent in a direction orthogonal to a direction, in which each of the presser plate portions 9c is extended, from the top ends of side surfaces of the leg plate portions 9b and engage the inner surfaces of the concave grooves 8, respectively, and a catch piece portion 9e, which is bent in a direction opposite to the direction, in which each of the presser plate portions 9c is extended, from the fastening device body 9a and faces the latter side surface 5d. Each of the leg plate portions 9b is fitted into a corresponding one of the concave grooves 8. Further, the fastening device body 9a is pushed into the light passing hole 2. Thus, the half mirror 3 is fixed in the light passing hole 2.
As illustrated in FIG. 9, in the apparatus of the aforementioned conventional configuration, the presser plate portion 9c is pressed against the half mirror 3 and elastically deformed by pushing the fastening device body 9a into the light passing hole 2. The half mirror 3 is pressed against and fixed to the slope-like top surfaces 6a of both the triangular plates 6 by a pushing force P that is generated owing to the elastic deformation of the presser plate portion 9c. A horizontal component F1 of a reaction force F generated by pushing the half mirror 3 in such a manner does not act in a direction in which this component F1 causes both the engaging claw portions 9d to bite into the inner surface portions of the concave grooves 8, respectively. Thus, a force for fixing the position of the fastening device body 9a in such a way as to prevent the body 9a from floating up is weak. Consequently, there is the possibility that the fastening body 9a is pushed up by a vertical component F2 of the reaction force F, so that the fixation of the half mirror 3 becomes unreliable.